Partly it depends on what you are trying to increase the size of... number of pages, number of images, size of a single image. In my experience, the vast bulk (90%+) of any given 'large' PDF file will be the images.
You could try using a pro product like Adobe InDesign to quickly build a large project and export it as a PDF.
Adobe Acrobat Pro has built-in tools to optimize PDF files -- you try using the tools to 'un-optimize' your file. :)